Smart Sensors

Process Technologies

(a). Bulk Micromachining Techniques:

Silicon etching using aqueous Tetra-Methyl Ammonium Hydroxide (TMAH) provides a simple and cost effective technique for the realization of structures/patterns for different applications. CSIR-CEERI has developed expertise in the field of wet bulk micromachining. The essential steps of this process are shown in Fig. 1. The process starts with thermal oxidation and LPCVD nitride deposition on a Silicon (100) wafer. The thermal oxide and silicon nitride are etched at appropriate regions after lithography. Next, wet bulk micromachining is carried out using TMAH to realize the structure. The oxide and nitride are etched subsequently. The etch rates in the {100} and {111} planes for 25% wt TMAH have been empirically estimated at different temperatures. These findings are extremely useful in the realization of structures with precise dimensions. Using TMAH-based wet bulk micromachining, the diaphragm and proof mass of pressure sensor and accelerometer have been realized. The SEM image of a MEMS pressure sensor diaphragm realized using this technology is shown in Fig. 2. The image of a MEMS accelerometer proof mass realized using this technology is shown in Fig. 3.

Figure 1: Wet Bulk Micromachining process

 Figure 2: SEM image of MEMS pressure sensor diaphragm

Figure 3: Image of MEMS accelerometer proofmass realized using TMAH

(b). UV-LIGA Technology: 

At CSIR-CEERI, we have developed an SU-8 based UV-LIGA process for the fabrication of MEMS inertial sensors. The essential steps of this process are shown in Fig. 4. The process starts with oxidation of a silicon wafer. Then, a gold layer is deposited and patterned to make the capacitor electrodes and the bonding pads. Again, a gold layer is deposited on the whole wafer surface as the seed layer. Following the patterning of a photoresist, a copper sacrificial layer is electroformed on the whole wafer surface except the anchor regions. Next, once again a photoresist is coated and patterned using the micro-gyroscope structure mask. Low stress nickel is electroformed inside this photoresist mold. Finally, the copper sacrificial layer and the gold seed layer are etched out selectively to release the structure. The SEM image of the MEMS gyroscope fabricated using this technology is shown in Fig. 5.

Figure 4: UV-LIGA process

Figure 5: SEM image of the fabricated MEMS gyroscope

(c). Wafer Bonding Techniques:

Wafer bonding techniques refer to the techniques in which two/three wafers are bonded together to realize the 3D microstructures of sensors/actuators using anodic/fusion bonding processes. Anodic boning technique is a field-assisted bonding technique, in which Pyrex glass wafer is bonded to the silicon wafer. Fusion bonding technique is a thermo-compression bonding in which homogeneous and heterogeneous materials can be bonded together. Fig. 6 shows the fabricated Capacitive Micromachined Ultrasonic Transducer (CMUT) arrays using anodic wafer bonding technique, and device under test using LDV (MSA500).

Figure 6: Capacitive Micromachined Ultrasonic Transducer Array under Test at LDV

(d). Materials Development:

Material development is an important aspect for realization of any reliable device. Amongst various actuation mechanisms, viz.; thermal, electrostatic and piezoelectric, development of piezoelectric material is very important and requires development of special process. CSIR-CEERI has developed piezoelectric zinc oxide (ZnO) thin film process technology, which includes ZnO film deposition, patterning and integration in MEMS/NEMS devices. The films are deposited using reactive sputtering method. Process parameters are optimized for the room temperature deposition which is highly desirable for MEMS device processing. Developed ZnO films are highly c-axis oriented with full width half maxima (FWHM) ~ 0.26˚, grain size ~80 nm and have minimal stress. AFM image of the ZnO thin film is shown in Fig. 6. SEM images of microcantilever array and Film Bulk Acoustic Wave Resonator (FBAR) fabricated using ZnO process technology are shown in Figure 7 and 8, respectively.

Figure 7: AFM image of ZnO film

Figure 8: ZnO thin film-based array of micro-cantilevers

Figure 9: FBAR using ZnO thin film

On-going Projects

  1. Development of MEMS Magnetic Sensor and RF/Microwave Tunable Devices (DRDO).
  2. Fabrication of Microlens for Laser Range Finder System (DRDO).
  3. Development of MEMS-based Accelerometer (CSIR-FTTP).
  4. Design and Development of Microcantilever-based Biosensor (DRDO).
  5. Design, Materials Development and Fabrication of Capacitive Micromachined Ultrasonic Transducer (CSIR-CNR).